As osmo-ionoconformers, hagfish gills are not involved in ionoregulation for the purposes of ion (i.e., Na+ and Cl-) balance as in other fishes and interestingly, their gills do not appear to play much of a role in gas exchange. Instead, 80–90% of oxygen uptake in hagfishes occurs across the skin at rest1011 due to the high epidermal SA:V characteristic of all extant or extinct early fishes (Fig. 1). Given the high dependence upon cutaneous respiration, perhaps it is not surprising that hagfish have the lowest oxygen consumption rate (MO2) of any craniate at rest or following severe stress1213. A few studies have provided evidence for acid-base relevant ionoregulation in the hagfish gill141516 by identifying associated gill transporters171819 and cell types with a recent study providing the first direct support for acid extrusion via the gills through use of a divided chamber system16. However, acid-base regulatory capacity (as informed by the rate and degree of pH compensation following a disturbance) in response to an environmentally induced acidosis (as opposed to acid injection) has not been specifically investigated for comparison with other aquatic craniates.